The present invention relates to a refrigerating oil composition for a refrigerator using a carbon dioxide (CO2) refrigerant and, more particularly, to a refrigerating oil composition which can be used for compression-type refrigerators using a carbon dioxide refrigerant, which are, specifically, refrigerating and air conditioning apparatuses such as automobile air conditioners, refrigerators, freezers, general use air conditioners and heat pumps.
In general, refrigerators such as compression-type refrigerators comprising a compressor, a condenser, an expansion valve and an evaporator has a structure in which a mixed fluid of a refrigerant and a lubricating oil is circulated in the closed system. Heretofore, chlorofluorocarbons such as dichlorodifluoromethane (R-12) and chlorodifluoromethane (R-22) have been used as the refrigerant for the compression-type refrigerators and various types of lubricating oils have been produced and used in combination with the refrigerant. However, since there is the possibility that these chlorofluorocarbons cause environmental pollution such as ozonosphere destruction in stratosphere when these substances are released into the atmosphere, the regulation on the chlorofluorocarbons is becoming stricter worldwide. Due to this situation, novel refrigerants such as hydrofluorocarbons and fluorocarbons, typical examples of which include 1,1,1,2-tetrafluoroethane (R-134a), are attracting attention. Although there is no anxiety that the hydrofluorocarbons and the fluorocarbons destruct the ozonosphere, there is the fear that these substances might cause global warming due to the longevity of these substances in the atmosphere. Therefore, the use of a natural substance-based refrigerant that does not cause these problems is considered.
Carbon dioxide is advantageous as the foregoing natural substance since it is harmless to the environment and safe to the human beings. Further, it is easily available anywhere as desired, it is not necessary to be recycled and it is very inexpensive. Therefore, carbon dioxide has heretofore been used as a refrigerant for refrigerators. However, the refrigerating system using the carbon dioxide refrigerant is a system having a higher pressure and a higher temperature than those of the refrigerating system using R-134a or the like and, moreover, is a system with transitional supercritical cycle having the supercritical condition in the refrigerant cycle. Therefore, when this system is lubricated with a lubricating oil which has heretofore been used conventionally, there is a great possibility of causing problems on lubrication in that deterioration in the lubricating property such as insufficient wear resistance takes place and stable use cannot be achieved for a long time due to a decrease in stability.
Polyoxyalkylene glycols have been used advantageously in the systems using the R-134a refrigerant and it is considered that polyalkylene glycols can be applied to the system using the carbon dioxide refrigerant. However, miscibility of the polyoxyalkylene glycols with carbon dioxide is not sufficiently great.
Under the foregoing circumstances, an object of the present invention is to provide a refrigerating oil composition for a carbon dioxide refrigerant that exhibits sufficient antiwear and excellent lubricating property, improved miscibility with the carbon dioxide refrigerant and that can be used for a long time with stability in the refrigerating cycle using the refrigerant comprising carbon dioxide in the supercritical condition of a high temperature and a high pressure as the main component.
As the result of intensive studies by the inventors, it was found that the object of the present invention could be effectively achieved by using a composition containing a base oil composition that comprises a polyoxyalkylene glycol with a specific kinematic viscosity, a specific amount of a carbonate-based carbonyl derivative and/or a polyol ester each having a specific kinematic viscosity. The present invention has been completed based on this finding.
The refrigerating oil composition for a carbon dioxide refrigerant as a natural substance-based refrigerants of the present invention has the following characteristics.
(1) A refrigerating oil composition for a carbon dioxide refrigerant containing a base oil composition which comprises (A) a polyoxyalkylene glycol having a kinematic viscosity of 3 to 50 mm2/s at 100xc2x0 C. and at least one component selected from (B) a carbonate-based carbonyl derivative having a kinematic viscosity of 3 to 50 mm2/s at 100xc2x0 C. and (C) a polyol ester having a kinematic viscosity of 3 to 50 mm2/s at 100xc2x0 C., wherein an amount of (B) and/or (C) is 0.1 to 40% by weight to the total base oil composition.
(2) A refrigerating oil composition for a carbon dioxide refrigerant of the description (1), wherein the base oil composition has a kinematic viscosity of 7 to 30 mm2/s at 100xc2x0 C. and a viscosity index of 130 or greater.
(3) A refrigerating oil composition for a carbon dioxide refrigerant of the foregoing description (1) or (2), which further comprises at least one acid catcher in an entire amount of 0.005 to 5.0% by weight.
(4) A refrigerating oil composition for a carbon dioxide refrigerant of the foregoing description (1) or (2), which further comprises at least one extreme pressure agent in an entire amount of 0.005 to 5.0% by weight.
(5) A refrigerating oil composition for a carbon dioxide refrigerant of the description (4), wherein the extreme pressure agent is at least one agent selected from a group consisting of metal salts of carboxylic acids and phosphorus-based extreme pressure agents.